Inkjet printing is becoming an increasing popular technique for the production of graphic images for advertising and the like. An inkjet printer comprises a printhead that includes a series of nozzles which eject ink onto a substrate. Two main ink chemistries are used in the art: inks that dry by exposure to ultraviolet radiation and inks that dry by solvent evaporation.
Inks that dry by exposure to UV radiation comprise a UV curable carrier that polymerises, or crosslinks, on exposure to UV light to form a crosslinked polymer film. These inks dry quickly, adhere to a wide range of substrates, and provide printed films with good solvent resistance. However, UV curable inks have relatively high viscosities and are therefore generally heated at the printhead in order to reduce the viscosity of the ink sufficiently so that the ink can be jetted. For example, a UV curable ink that has a viscosity of 20 to 22 mPas at 25° C. would be heated to around 38 to 45° C. at the printhead in order to reduce the viscosity to less than 10 mPas. This requirement for heating adds complexity to the printhead which means that currently available printheads for use with UV curable inks are relatively expensive and more complex engineering is required to achieve the desired reliability. Printheads that are adapted to print this type of ink include the Dimatix SE128 and the Toshiba Tec CA4 drop-on-demand piezo print heads.
Because of their intrinsically low viscosity, solvent-based inkjet inks do not need to be heated in order to be jetted satisfactorily at the inkjet printhead; indeed some lower cost printheads simply cannot be heated. The printheads used to print solvent-based inkjet inks are therefore typically lower cost, such as the drop-on-demand piezo print heads made by Epson. However, current solvent-based inkjet inks may not adhere to certain types of substrate, particularly non-porous substrates such as plastics, and the printed films have poor resistance to solvents.
There therefore exists a need for an inkjet ink that has good solvent resistance and that can be printed on a wide range of substrates, but which can be printed without the need for heating at the printhead and can therefore be used with currently available, low cost printheads.